[82.03] The Transience of the 1612 MHz Emission Phase of OH/IR Stars

B. M. Lewis (Arecibo Observatory)

There are 69 OH/IR stars with \vert b \vert > 10\deg and a 25 {\mu }m flux S(25) > 2\,Jy in the complete
Arecibo 1612\,MHz survey of color-selected IRAS sources. WX
Psc is one. There are also \ge4 (likely 6) proto planetary
nebulae with these parameters. As OH/IR stars evolve into
PPN, the expansion age of one, 18095+2704, at \approx
110\,yr gives their time scale. Thus the net duration of
their 1612\,MHz emission phases is (69/6 + 1) *
110\,\approx \,1400\, yr. This is comparable with a wind
travel-time ({\Delta }t c/2\, ve) \approx 770\,yr
from the photosphere to the 1612\,MHz emitting region, which
derives from the phase- lag between the blue and red peaks
of WX Psc (van Langevelde et al. A&A 239, 193). It is also
comparable to the {~}500\,yr duration of the
luminosity spike following on a thermal pulse in the host
AGB star, as energy from its He-shell flash is radiated
(e.g. Wood & Zarro ApJ 247, 247). Our objects also fall
into four clumps on a ve-color plot, which suggests
they come from 3-5 thermal-pulse cycles. The emission phase
in any one cycle is thus \ll500\,yr. Most are only likely
to exhibit masers while the luminosity spike decays.

A similarly transient emission phase for all OH/IR stars is
implied by the well-known weakness of CO emission from OH/IR
stars in general: steady state models predict
rCO\,\approx \,10\,*\,rOH, a state rarely if ever
achieved. Likewise the existence of OH/IR star color mimics,
objects which on the basis of their IR colors should exhibit
1612\,MHz masers but do not (Lewis ApJ 396, 251), follows as
a mandatory result of the abrupt onset of the superwind. The
consequent rapid increase in ve causes the shell to
outstrip its prior dust distribution, which drastically
shortens the longevity of OH molecules in the new expanding
shell until adequate dust shielding against interstellar UV
is again in place at much larger radii. Thus most mimics are
a natural consequence of the onset of a superwind, and their
high frequency (>40%) among color- selected sources
points to the transient nature of the 1612\, MHz emission
phase.

The author(s) of this abstract have provided an email address
for comments about the abstract:
blewis@naic.edu